1. Field of the Invention
The present invention relates to a structure of a speaker apparatus applicable to a vehicle which improves the audible frequency characteristic of the speaker apparatus.
2. Background of the Art
Japanese Utility Model Application First Publication (non-examined) Sho 55-165570 exemplifies a speaker structure.
The structure of the speaker apparatus disclosed in the above-identified Japanese Utility Model Application Publication includes an opening defined by a side surface of a lower casing and a speaker unit incorporated into the lower casing so as to face upward. In addition, an upper casing covers the speaker unit and lower casing. The upper casing is in a non-bottom shape and has another opening at a side surface opposite to the opening of the lower casing.
Thus, a cabinet of the speaker is constituted by the lower casing and upper casing with a ceiling of the lower casing as an intermediate partitioning wall.
In the structure of the speaker described above, a ceiling of the upper casing opposes a front surface of a vibrating plate of the speaker unit. An interval distance between the ceiling of the upper casing and vibrating plate is normally set about to 3 cm.
Therefore, a sound radiated toward a front surface of the vibrating plate repeats its reflection so that a resonance occurs having a frequency f expressed in the following equation. EQU f=C.times.n/2 l (C denotes sound speed, l denotes an interval of distance between the vibrating plate and ceiling, and n=1,2,3, - - - ).
Hence, the sound radiated from the opening of the upper casing is caused to generate a peak and dip at a frequency above about 7.5 KHz and generate a acoustical distortion even in a band about half of the frequency. Consequently, a flat frequency characteristic, an ideal frequency characteristic provided by the speaker apparatus cannot be achieved.